Urban sediment contamination with heavy metals and organic matter in Southern China

Jinmeng Xia^b, Jianbing Li^b,c

^a Chinese Research Academy of Environmental Sciences, Beijing, 100012, China

^b College of Life and Environmental Sciences, Wenzhou University, Wenzhou, 325035, China

^c Environmental Engineering Program, University of Northern British Columbia (UNBC), Prince George, BC V2N 4Z9, Canada

Guifang Liu(liuguifang@126.com)

Sheta M Sheta(dr.sheta.nrc@gmail.com)

Xufeng Fei(feixf@mail.zaas.ac.cn)

Cristian Simion(c_simion@chim.upb.ro)

Jinmeng Xia, Jianbing Li, Urban sediment contamination with heavy metals and organic matter in Southern China(2019)Journal of Earth Sciences & Environmental Studies 4(3)

Urban sediments in Wenzhou City were found polluted with heavy metals (As, Hg, Cr, Pb, Cu, Cd, and Zn) and organic matter. Metal speciation also showed environmental risks of heavy metals in river sediments. Heat energy in the sediment can amount to 47% of that in standard coal. Investigations revealed 35 urban wetlands of 15 big cities in southern China were at ecological risks posed by heavy metals and excessive nutrients in sediments. Toxic metals and organic carbon must be taken into consideration when planning to project urban wetlands.The dredging of such sediments could possibly increase greenhouse gases emission that should be further investigated. It is also suggested to examine energy recycling from these wetland sediments.

KEYWORDS Urban sediment, Heavy metals, Organic carbon, South China, Greenhouse gas emission

With rapid economic development for 4 decades, wetland sediments in most cities in China have been polluted at different levels by heavy metals [1,2]. Sediments are important storage sites of heavy metals and the potential sources of pollution. The Chinese tradition that sediments are dug from human impacted rivers and then used as fertilizers for farming can be traced back thousand years ago. Metals and metalloids accumulated in sediments pose environmental problems concerning metals being transferred from solid phases to the aquatic medium and thereby included in the food chain [3].

Urban wetlands play an important role in maintaining the environmental systems of cities in southeastern China where population is large, economy is active and rainfall is plentiful. These wetlands accept discharges from residential houses, factories, schools, offices, farms and rainwater which lixiviate and scour land surfaces, thus leading to the accumulation of organic and inorganic matters in the sediments. The fate of organic carbons in sediments has become a challenging issue, especially when they are exposed to air after dredging. The present study is then to investigate organic carbon in an urban wetland in Wenzhou City, southeastern China, while the implication on potential greenhouse gas emission from urban sediment is also discussed.

We analyzed 31 sediment samples collected from 2 representative rivers (21 samples from R_A and 10 for R_B) in the urban area of Wenzhou, a prefecture city in southeastern China to assess contamination by heavy metals and organic matter. Total contents of heavy metals (As, Hg, Cr, Pb, Cu, Cd, and Zn) and their chemical forms were determined by using sequential extraction [6]. Ecological Risk Index (RI) [4] and Risk Assessment Code (RAC) [5]were employed to evaluate ecological and environmental risks of the rivers impacted by heavy metals. Ratio of second phase (the sum of exchangeable, oxidible, reducible fractions, labeled F1, F2 and F3)to primary phase (residual fraction, F4) (RSP) was adopted to describe metal speciation in sediment [3,7]. Total contents and chemical forms of organic carbon were analyzed by using potassium dichromate sulfuric acid spectrophotometry [8]) and ¹H NMR methods[9].  Organic index (calculated by the multiplication of carbon fraction and nitrogen fraction) [10] was employed to assess organic pollution level.

We also consulted literature published in Chinese on toxic metals in wetlands impacted by big cities in south China, giving a big picture of heavy metal and organic pollution in urban sediments. Potential greenhouse gas emission and recycling of organic - rich sediment in urban areas of representative big cities in China was also discussed.

3.1 Heavy metals and risks of urban sediments in Wenzhou

3.1.1 Total contents and RI

The mean values of total contents of heavy metals (mg g-1) in the R_A was in the order of Zn (810.46, ±292.94) > Pb (228.73, ±57.67) > Cr (129.14, ±48.14) > Cu (102.66, ±46.71) > Cd (101.56, ±18.07) and R_B was Zn (1193.45, ±403.12) > Cu ( 332.40, ±180.34) > Cr (300.80, ±83.93) > Pb (97.25, ±21.06) > Cd (9.64, ±2.66). And the values of RI in R_A and R_B were (18208.53, ±3235.69) and (268.77, ±325.41), respectively. The contamination of heavy metals in the sediments of R_A ( High risk ) was more serious than that in the R_B ( Considerable Risk ).

3.1.2 RSP and RAC

Total metal content in polluted environmental samples is a poor indicator of bioavailability, mobility or toxicity and all of these depend on the different chemical forms of binding between trace metals and solid phases of the samples. The order of RSP in R_A was Zn (7.65, ±4.45) > Cd (2.93, ±0.84) > Cu (2.00, ±0.83) > Pb (1.46, ±0.47) > Cr (1.11, ±0.46), while the order of that in R_B was Cd (3.64, ±1.41) > Pb (0.63, ±0.13) > Cu (0.60, ±0.20) > Zn (0.37, ±0.10) > Cr (0.30, ±0.06). According to RAC, RA were assessed Zn, Cu, and Cr at low risk, Pb at medium risk and Cd at high risk, while R_B were assessed Cu and Cr at low risk and Zn, Pb, and Cd at Medium risk.

3.2 Heavy metal risks of urban sediments in south China

Thirty five urban wetlands in 15 big cities in southern China were contaminated by heavy metals according to RI assessments Urban rivers were closer to industrial and residential facilities than lakes, consequently being more impacted. Nineteen rivers were assessed 6 at High Risk, 9 at Considerable, 1 at Moderate and 3 at Low Risks, while lakes were 11 at Low and 5 at Moderate Risks (Table 1 and 2).

3.3 Organic carbon of urban sediment in Wenzhou

The contents of surface sediment organic carbon in Sanyang Wetland in Wenzhou ranged from 17.94 to 50.57mg/g, with an average value of 31.91mg/g and a standard deviation of ±17.50mg/g, and the heavy organic carbon fraction accounted for 97.44- 98.71% of the total organic carbon. The mean ratio of C/N was 13.64 with a standard deviation of ±0.78. The carbon fraction measured by 1H NMR was ranked as follows: Aliphatic group > Alkoxy group > Phenyl > Carboxyl > Acetal group, with a value of 38.38%, 21.79%, 18.08%, 18.03% and 3.4%, respectively. The high oxidation of organic carbon in sediments implied human impacts and anthropogenic sources from terrestrial areas.

The organic indexes of 9 urban wetland sediment samples ranged from 0.2413 to 1.769, with an average of 1.124, falling into the highest level Ⅳ. The results indicate that the sediments were highly impacted by anthropogenic activities.

3.4 Organic carbon in urban sediments of southeastern Chinese cities

3.4.1 Organic carbon contents

Total organic carbon of 17 selected urban wetlands in southern China was averaged at 39.77g/kg with a range from 2.68 to 129.0g/kg. These cities (including 9 provincial capitals) all have a population of over 2 million, and their wetlands have been polluted with organic matter. Fourteen of these selected urban wetlands had organic pollution level of Ⅲ and Ⅳ, and other three had pollution at level Ⅱ.

3.4.2 Global change issues related to ex situ disposal of polluted sediment

With increasing contamination of urban wetlands and heightening awareness of environmental protection, many cities in China have been or are planning dredging polluted sediments for better wetlands. In south China, urban wetlands are in big number and cover large areas of land, resulting in huge volume of polluted sediments. For example, Wenzhou is a coastal prefecture city with about 2 million of population and 910 km² of terrestrial land. Its three urban districts have 229 km² of water bodies (urban rivers, lakes and ponds), among which Sanyang Wetland has received increasing attention. Oure previous results indicated that there was 1.4 million m³ of organic polluted sediments in 3.2 km² of rivers in this wetland[11]. The municipal government of Wenzhou has implemented the remediation and restoration of polluted rivers since 2005. However, the exposure of organic rich sediments to air after dredging led to various concerns because the oxidation of organic matter may increase the emission of carbon dioxide and other greenhouse gases.

3.4.3 Energy recycling

Organic polluted sediments contain valuable heat energy. The calorific values of sediments were 13010 - 18481 kJ/kg (i.e. about 47% of that of standard coal) for Sanyang Wetland and 7000 – 10000kJ/kg for Dianchi Lake [12]. It is thus possible to recycle heat energy from polluted sediment as a substitute of coal. The sediments can also be used for the production of brick or ceramics, honeycomb briquettes and so on.

Table 1  Heavy metal contents (mg g-1)and Potential Ecological Risk Index (RI) of 35 urban wetlands in 15 big cities, South China

*Fifteen cities : 2 Province level (Chongqing’ Shanghai); 6 Provincial capitals (Changsha, Hangzhou, Hefei, Nanchang, Nanjing,Wuhan); and 7 Prefecture cities (Dongguan, Hengyang, Hengshui, Ningbo, Shenzhen, Suzhou, Wenzhou)

Table 2   Organic carbon contents and organic indexes of urban wetland sediments in 17 cities, southeastern China

Two representative rivers (RA and RB) in the urban area of Wenzhou City were found contaminated by heavy metals. RA and RB were assessed High Risk and Considerable Risk respectively by RI. And RSP and RAC indicated contamination of urban sediments in Wenzhou with high risk of Cd in the RB and low and medium risks of other metals in both RA and RB.

Thirty five urban wetlands in 15 big cities in southern China were assessed 15 at High and Considerable and 20 at Moderate and Low risks. Urban sediments can no more be used as fertilizers or soil amendments applied to croplands and any other soils.

Sediments of Sanyang Wetland in Wenzhou have been polluted by organic matter with organic carbon content up to 50 mg/g. Aliphatic group accounted for 38% of the total organic carbon, presenting high oxidation level. Seventeen urban wetlands in the big cities in southern China were evaluated for their pollution with organic matter, and the results indicated that urban sediments were rich with organic carbon. Improper disposal of such polluted sediments can accelerate organic matter oxidation and consequently increase the emission of greenhouse gases. It is suggested to further research into polluted sediments as a carbon sink and a greenhouse gas source. Furthermore, the high caloric value of urban sediments could lead to great potential for energy recycling, and the feasibility to utilize these organic rich sediments as a potential energy needs further investigation and validation.

1. X. Zhu, H. Ji, Y. Chen, M. Qiao, L.Tang. Assessment and sources of heavy metals in surface sediments of Miyun Reservoir. Environmental Monitoring and Assessment., 2013,185: 6049-6062 PMid:23208758

   View Article      PubMed/NCBI     

2. J. Bai, B. Cui, K. Zhang, W. Deng, H. Gao, R. Xiao. Spatial distribution and ecological risk assessment of heavy metals in surface sediments from a typical plateau lake wetland. Ecol.Model., 2011,222: 301-306

   View Article           

3. K. Nemati, N.K.A. Bakar, M.R. Abas, E. Sobhanzadeh. Speciation of heavy metals by modified BCR sequential extraction procedure in different depths of sediments from Sungai Buloh. Journal of Hazardous Materials., 2011,192: 402-410

   View Article           

4. L. Hakanson. An ecological risk index for aquatic pollution control. A sedimentological approach. Water Research., 1980, 14:975-1001 90143-8

   View Article           

5. C. K. Jain. Metal fractionation study on bed sediments of River Yamuna. Water Research., 2004,38:569-578 PMid:14723925

   View Article      PubMed/NCBI     

6. M. Ghaedi, F. Ahmadi, M. Soylak. Preconcentration and speciation of nickel,copper and cobalt using solid phase extraction and their determination in some real samples. J.Hazard. Mater., 2007,147: 226-231 PMid:17303327

   View Article      PubMed/NCBI     

7. C. C. Wang, Y. Liu, P. Wang, C. Z. Wang. Characteristics of Sediments Polluted by Heavy Metals in a Reach of Tonghui River. Environmentals Science & Technology., 2011, 34(7):186-190

8. Z. H. Guo , L. Zhang, Y. R. Guo, W. Y. Wen, M. Cao, J. L.Guo, Z. Y. Li. Soil Carbon Sequestration and Its Relationship with Soil pH in Qinglangang Mangrove Wetlands in Hainan Island.Scientia Silvae Sinicae., 2014;8-14

9. Y. Lin.Isolation and Characterization of Reference Organic Matterfrom Soil,Sediment,and Blue Algae.Bei Jing, 2011

10. H. Yu, W. B. Zhang, S. Y. Lu, S. W. Yan, R. J. Hu, L. Chen, L. L. Zhang, J. p. Yu. Spatial Distribution Characteristics of Surface Sediments Nutrients in Lake Hongze and Their Pollution Status Evaluation., 2010:961-968

11. Y. B. Li, W. Di, W. T. Ronald, D. D.& J. B. Li. Bathymetric modeling of sediments and organic carbon of polluted rivers in southeastern China. J Soils Sediments., 2016:2296-2305

    View Article           

12. J. R. Zhao. Pyrolysis products characteristic of Dianchi Lake sediment and desulfurization performance of Dianchi lake sediment supported catalyst. Kunming University of Science and Technology. Kun Ming, 2015